THE PARAFFIN METHOD 
111 
The grades of alcohol may be used several times, but it must be 
remembered that pollen grains, fungus spores, starch grains, and vari¬ 
ous granules are likely to be left in the alcohol, so that it is wise to 
pour back through a filter each time, thus keeping the alcohols clean. 
As the alcohols absorb water from the material, they become 
weaker and weaker. If the various alcohols be poured in a large 
“waste alcohol” bottle, when a couple of liters has been accumulated, 
the strength may be determined by testing with an alcoholometer. 
Then any grade of less strength can be made from this stock. 
The time necessary for each of the stages has not been determined 
with any certainty. About 2 hours seems to be long enough for 
each of the grades from to 70 per cent; for 70, 85, and 95, about 
4 hours each; for absolute alcohol, 4 to 12 hours, changing 2 or 3 
times. If material is to be kept in alcohol, leave it in 85 per cent, 
but where labor is no object, it is better to go on and imbed 
it in paraffin. 
CLEARING 
Let us suppose that the material has been thoroughly dehydrated, 
so that not the slightest trace of water remains. If the supposition 
chances to be contrary to fact, all the work which has preceded, as 
well as all which is to follow, is only an idle waste of time. The 
purpose of a clearing agent is to make the tissues transparent, but 
clearing agents also replace the alcohol. At this stage the latter 
process is the essential one, the clearing which accompanies it being 
incidental. The clearing, however, is very convenient, since it 
shows that the alcohol has been replaced and that the material is 
ready for the next step. 
Various clearing agents are in use. Xylol is the most generally 
employed, and for most purposes it seems to be the best. Bergamot 
oil, cedar oil, clove oil, turpentine, and chloroform are used for the 
same purpose. Cedar oil and chloroform may, in some cases, be as 
good as xylol. 
Only a small quantity of the clearing agent is necessary, enough 
to cover the material being sufficient. 
The transfer from absolute alcohol to the clearing agent should 
be gradual, like the hardening and dehydrating processes. The most 
successful workers have been making this transfer more and more 
gradual. Thirty years ago it was customary to transfer from absolute 
alcohol directly to xylol; then a mixture of equal parts of absolute 
